The present disclosure generally relates to devices and methods for generating and delivering continuous positive airway pressure therapy or other non-invasive breathing assistance to patients, such as infants. More particularly, the present disclosure relates to variable flow, nasal continuous positive airway pressure systems, devices, and methods with reduced driving pressure requirements and improved work-of-breathing.
Continuous positive airway pressure (CPAP) therapy has been employed for many years to treat patients experiencing respiratory difficulties and/or insufficiencies. In addition, CPAP therapy can beneficially assist patients with under-developed lungs (in particular, infants and especially premature infants or neonates) by preventing lung collapse during exhalation and assisting lung expansion during inhalation.
In general terms, CPAP therapy entails the continuous transmission of positive pressure into the lungs of a spontaneously breathing patient throughout the respiratory cycle. CPAP can be delivered to the patient using a variety of patient interface devices, for example an endotracheal tube or nasal cannula. With infants, however, it is more desirable to employ a non-invasive patient interface device, in particular one that interfaces directly or indirectly with the nasal airways via the patient's nares. Such systems are commonly referred as nasal continuous positive airway pressure (nCPAP) systems.
In theory, the CPAP system should deliver a constant, stable pressure (above atmospheric pressure) to the patient's airways. With conventional CPAP systems, a relatively constant and continuous flow of gas (e.g., air, oxygen, etc.) is delivered into the patient's airways, with this airflow creating a pressure within the patient's lungs via a restriction placed on outflow from the patient. Unfortunately, this continuous flow can have an adverse effect on the patient's respiratory synchrony. More particularly, the patient is required to exhale against the incoming gas, thus increasing the patient's work-of-breathing. Control valves can be employed to better accommodate inspiratory and expiratory stages of a patient's breathing cycle (e.g., controlling gas flow into the system and/or altering an extent of restriction from outflow from the system). However, for many patients, especially infants, this approach is less than satisfactory as the patient's required work-of-breathing is quite high. That is to say, it is essentially impossible for a control valve system to accurately replicate the actual respiratory cycles experienced by the patient, such that the patient will consistently be required to exhale against the momentum of the incoming gas, as well as against the resistance of the control valve(s). For an infant with underdeveloped lungs, even a slight increase in the required work-of-breathing may render the CPAP system in question impractical.
More recently, nCPAP systems have been developed that incorporate a variable flow concept in combination with separate channels for inspiratory and expiratory gas to and from the patient. When the patient inhales, the incoming gas takes the path of least resistance and is directed to the patient's airways. Upon expiration, the gas again takes the path of least resistance and goes out an exhaust port, thus reducing resistance during the expiratory phase of breathing. For example, the Infant Flow™ system, available from CareFusion, Inc., of San Diego, Calif., includes a variable flow CPAP generating device (or “CPAP generator”) that causes the direction of the supply gas to change with the infant's breathing patterns while maintaining a nearly constant pressure throughout the respiratory cycle. The Infant Flow CPAP generator converts supplied gas into jet streams (one for each naris), with the momentum of the gas jet creating a positive pressure inside the patient's lungs, in accordance with known physics principles. To accommodate expiratory flow from the patient, the Infant Flow CPAP generator relies upon what the manufacturer's literature lists as a “fluidic flip” effect. The expiratory airflow from the patient applies a pressure onto the incoming jet steam flow. It has been theorized that due to the Coanda effect, the expiratory airflow causes the jet stream flow to deflect, thus triggering a fluidic flip of the incoming jet flow. As a result, the jet stream and the expiratory airflow readily proceed to the exhaust port, thus reducing the patient's required work-of-breathing. While quite promising, the jet streams generated in such devices have a relatively high momentum that may not be easily overcome by the patient's expiratory breathing, especially with infants. Moreover, driving gas pressure levels that must be applied to these and other commercially available variable-flow CPAP generators to produce therapeutic CPAP levels are not sufficiently low to permit usage with a common ventilator. Instead, a dedicated high-pressure flow driver is required.
In light of the above, needs exist for improved nCPAP systems, devices, and methods.